Adaptation To Habitats With Limited Resources

Avoidance or tolerance is probably the most apt description of the principal survival strategies that are associated with plants in marginal areas. Either the plants adapt to reduced access to light, nutrients, water or oxygen, or else they have some means of avoiding acute deprivation of these essential resources. Low growth rates and a diminutive size are the commonest responses. Genotypic variation resulting in populations adopting this minimalist approach is often particularly evident in the distinct local races or ecotypes that have evolved in many marginal populations. Phenotypic plasticity can also produce dwarf plants. In some cases this reduction in growth is considered as merely a passive response due to a lack of resources. However, it may also be accompanied by other changes which suggest that even what appears to be a passive response may actually have a genetic basis. The dwarf and pubescent specimens of the sea plantain (Plantago maritima; Fig. 3.6) that are often found on sea cliffs, if transplanted to a less stressful habitat not only become much larger, but also usually lose their hairy leaves. However, not all coastal populations of sea plantain are hairy, which suggests these cliff-top populations are genetically diverse.

In relation to reduced light, plants vary in their response depending on whether they tolerate this condition by showing reduced growth or else attempt to escape from shade by the opposing strategy ofincreased growth as in shoot extension (etiolation). In these cases where the response is an attempt to escape from shade there has to be a high probability that increasing growth will achieve access to improved illumination otherwise the response will be maladaptive and decrease the probability of survival. Extension growth is frequently observed in aquatic plants where rooted macrophytes respond to increased submergence by stem and petiole extension (Voesenek et al., 2004). In submerged aquatics

Fig. 3.6 Hairy diminutive form of the sea plantain (Plantago maritima) growing on a cliff top in Orkney. This hairy form occurs along with glabrous forms on areas exposed to salt spray. However, when transferred to the shelter of a glass house the plant can grow larger and lose its pubescence.

the extension is generally hormone controlled. For terrestrial plants shade detection, leading to shoot extension, is mediated by sensing the change in the quality oflight, particularly the ratio of red to far red as perceived via phytochrome, or other photoreceptors.

Both phenotypic and genotypic variation can be examined in relation to their physiological responses to environmental conditions under headings which highlight metabolic activity. The first is capacity adaptation, in which alterations in specific morphological and physiological properties enable the plants to increase their capacity to carry out existing metabolic reactions under suboptimal conditions. The second is functional adjustment, which describes changes in behaviour (e.g. phenology) or the adoption of different physiological mechanisms as a response to alterations in environment.